The invention concerns a module support structure for printed circuit boards which can be inserted on guide rails and having electrical or electronic components. The module support structure comprises two parallel side walls and at least four parallel module rails connecting the side walls and bearing the guide rails.
Electronic circuits, in particular digital ones, have increasingly lower switching thresholds and are sensitive to interfering ambient radio frequency fields. For this reason, module support structures having printed circuit boards bearing this kind of sensitive circuitry must be shielded at all sides. Towards this end, one uses radio frequency sealed side walls and upper and lower sheet metal as well as rear covers. Shielding with respect to interfering radio frequency fields in the region of the front plate is, however, more problematic since individual printed circuit boards should be removable at any time. Particularly when the module support structure does not have a common front plate extending over the entire width of the module support structure, rather accepts plug-in modules having the inserted printed circuit boards mounted to individual front plates, the gaps between the front plates and the module support structure cause special problems, particularly at the module rails.
Shielding of the vertical gaps between the individual front plates and between the sideward front plates and the side walls is usually effected using resilient elements as e.g. described in the patent publication DE 41 10 800 C1, due to the applicant. Herein, resilient metallic elements are placed on the legs of a U-shaped front plate which each press against the neighboring front plate or against the side wall of the module support structure when the plug-in module is inserted to thereby effect the required low resistance area contact.
Differing approaches have been taken for shielding the horizontal gap between the front plates and the cover and floor sheets. The cover and floor sheets are usually contacted to the four module rails which each seat in the corners of the side walls and connect the side walls to each other. A low resistance contact is simultaneously provided between the front plates and the two front module rails to effect complete over-all shielding.
Known in the art from DE 41 26 576 A1 is a module support structure with module rails having a groove opened towards the front side of the module support structure for accepting a threaded-hole-strip and at least one seating surface for the front plate. Resilient elements are provided for which are inserted into the open grooves of the module rails and seat in widened portions of the grooves in front of the threaded-hole-strip. These resilient elements have an angled strip which protrudes beyond the grooves of the module rails in the forward direction and is pushed-in towards the threaded-hole-strip by the front plates in opposition to a resilient elastic force.
A contact element is known in the art from DE 296 02 426 U1 which seats in the groove, opening towards the front, of a module rail and is disposed in front of the threaded-hole-strip. The contacting is thereby effected via equidistantly disposed contact spikes projecting in a forward direction beyond the module rail.
One must however assure, when contacting the front plates as well as the cover and floor sheets to the module rails of the module support structure, that electrical contact is not impaired by oxidation layers. This is the case both for direct contact between the components as well as when utilizing an intermediate resilient element.
Module rails usually consist essentially of extruded aluminum profiled structures which are mechanically post-processed through sawing, punching or drilling. The naturally occurring oxide layer of untreated aluminum is very thin so that the surface of extruded aluminum profiled structures is unavoidably scratched during mechanical post-processing. Moreover, the naturally occurring oxide layer cannot prevent further irregular oxidation of the untreated aluminum profiled structures during transport or storage so that a generally undesirable surface is formed. In order to counteract this problem, the profiled aluminum structures are usually anodized directly following their manufacture. An anodized surface can be mechanically post-processed without scratching.
The electrical conductivity of a contact to aluminum having a naturally developed oxide layer is adequate for shielding against radio frequency alternating fields. The transition resistance of an anodized aluminum surface is, however, too large. For this reason, two differing types of module support structures have been provided for in prior art: module support structures without shielding having anodized aluminum module rails, and shielded module support structures with which, following mechanical post-processing, the originally anodized module rails are completely chemically freed from their anodized layer in a caustic solution. This required double storage by the module support structure manufacturer with regard to the module rails. Moreover, it is nearly impossible to retroactively shield a module support structure having anodized module rails.